Modern biomedical and environmental research and applications depend on detailed and comprehensive characterization of complex samples. The demands of specificity, sensitivity, and speed have made mass spectrometry (MS) the prevailing platform for such analyses. Most real samples are sufficiently challenging to necessitate one or more separation steps prior to MS. These separations are typically performed in the condensed phase, using liquid chromatography (LC) or capillary electrophoresis (CE). Nowadays, those methods are increasingly replaced or supplemented by separations in gases relying on ion mobility spectrometry (IMS), including field asymmetric waveform IMS (FANS).
MS can analyze ions only. For large and fragile molecules including proteins, peptides, DNA strands of significant length, and most metabolites and other biomolecules, electrospray ionization (ESI) and its derivatives such as desorption ESI or laser ablation ESI are commonly employed. The ESI efficiency is maximized at high (near-atmospheric) gas pressure and drops with decreasing pressure to zero in vacuum, hence ESI sources are normally operated at ambient pressure. Some ion sources, for example matrix-assisted laser desorption ionization (MALDI), can perform in vacuum, but are often employed at ambient pressure for speed and convenience. Use of such atmospheric pressure ionization (API) sources inevitably creates the problem of effective ion transfer into the MS vacuum through a necessarily narrow orifice that is typically much smaller than the produced ion swarm. The same issue arises when coupling IMS or FAIMS stages among themselves or to MS, where ion beams or packets that spread (because of diffusion and Coulomb repulsion) during separation must be introduced into an MS or another IMS stage via a narrow aperture.
In API/MS systems, the MS inlet has typically been fashioned as a curtain plate/orifice assembly (FIG. 1a) or a heated capillary (FIG. 1b). These differ in how the solvated ions generated by ESI are desolvated: by gas counter-flow while being pushed forward by an electric field (FIG. 1a) or heated gas flow (FIG. 1b). In either case, the conductance limit between the atmosphere and MS vacuum is much narrower than the incoming ion plume, leading to major ion losses even with a single ESI emitter. Losses are larger yet with emitter arrays that provide more effective and uniform ionization at lower liquid flow per emitter, but deliver ions over a wider area (FIG. 1c). The typical pressure in the first MS chamber after either interface is several Torr, the maximum for effective evacuation by standard vacuum pumps. Thus the gas coming from atmosphere supersonically expands, greatly broadening the ion beams beyond the aperture of the skimmer leading to the next MS chamber, which causes further losses. Thus ˜1% and often much less of ions produced by ESI are transmitted to the high-vacuum MS regions, limiting the MS sensitivity and dynamic range. Similarly, in drift-tube (DT) IMS, ion packets expand orthogonally to the tube axis during separation, and <1% of ions enter the following MS stage via a pinhole at the tube terminus (FIG. 1d). In conjunction with losses at the tube front and low DTIMS duty cycle, that has reduced sensitivity so severely as to preclude commercialization of DTIMS/MS systems and their use in most practical analyses. For FAIMS devices, the analytical gap geometry is crucial, Units with curved gaps feature an inhomogeneous electric field that focuses ions to the median. With hemispherical caps, those units produce tight beams that can pass through narrow MS inlets with few losses. This focusing also constrains the FANS resolving power, obstructing many applications. Planar FAIMS units have a homogeneous field that effects no focusing and thus may provide exceptional resolution, but ions freely diffuse, broadening the beam in the plane of the gap cross-section. In transverse-cylindrical FAILS units, ions are focused to the gap median but also freely diffuse in the lateral direction. Extracting such broadened beams through standard inlets to an MS (or reduced-pressure IMS) stage is associated with huge ion losses that limit the utility of high-resolution FAILS (FIG. 1e). Slit-aperture MS inlets that better match the rectangular cross-section of ion beams exiting planar FAIMS devices provide some improvement, but large losses remain.
The need to focus ion beams or packets at substantial gas pressure for transmission into lower-pressure instrument stages through a necessarily tight aperture is broadly encountered in MS and hyphenated MS, and is often critical for successful analyses. This need has previously been addressed using electrodynamic ion funnels, at the simplest comprising stacks of electrodes separated by insulator gaps (including air gaps) of given gap width (g) with circular apertures that narrow along the stack (FIG. 2a). An RF voltage of some frequency (w) and peak amplitude (U) applied to adjacent electrodes with opposite phases produces an oscillatory electric field near the funnel avails. The peak field intensity (A) rapidly drops when distancing from the walls, and the resulting Dehmelt potential repels ions toward the funnel axis, preventing their loss on the electrodes. A ladder of DC voltages is typically co-applied to electrodes to establish a potential gradient along the axis, which pulls confined ions through the funnel while compressing them to the diameter of the smallest exit aperture (d). In practice, the RF voltage is loaded onto the electrodes using two capacitor chains, one connected to the even-numbered electrodes and the other to the odd-numbered electrodes, and DC voltages are produced using a resistor chain. A pressure drop behind the funnel produces the vacuum suction and thus axial gas flow that accelerates toward the exit (FIG. 2a). This gas flow aids the DC field to pull ions along the funnel, and, depending on the funnel length, conical angle, and other design and operational parameters, may suffice to pull a large fraction of ions through the funnel even with no DC field. If the apertures narrow enough in terms of the electrode spacing (s), the RF field also creates axial traps that capture ions and impede their motion through the funnel. This effect rapidly grows as d decreases below 2 s, limiting the minimum practical final beam diameter to ˜1.5 s-2 s. The entrance opening is not physically restricted and should be large enough to collect most or all of the incoming ions. A 1-in. diameter has sufficed for ions expanding from as al inlet at the front end of MS or IMS stages. The funnels at DTIMS termini may need a larger opening, depending on the tube length, drift voltage, and gas temperature that control the ion expansion in the tube, and a 2-in. diameter has been used with longer tubes.
The base funnel implementation transmits incoming ions without significant delay, which is suitable for coupling to MS and has been broadly adopted to interface ESI, conventional IMS, and FAIMS units to various MS systems. However, DTIMS accepts ions in pulses and thus strongly benefits from ion accumulation before the starting gate. This need has been addressed using “hourglass” ion funnel traps (IFT) that comprise sections where apertures broaden along the direction of ion travel (FIG. 2b), providing the ion storage volume at a reduced pressure equal, or close, to that in the following chamber. on packets injected into the tube may be refocused (e.g., for better IMS resolving power) employing a “double hourglass” IFT that comprises another section of narrowing apertures (FIG. 2c). Such funnels are equally appropriate with DTIMS in the multiplexed mode and can work with any stage requiring pulsed ion introduction.
Non-accumulating funnels can transmit close to 100% of ions, at least at not-too-high flux where Coulomb repulsion is limited. “Hourglass” IFTs also have high ion utilization efficiency until the charge capacity is reached. For API/MS interfaces, the transmission through the inlet is roughly determined by the ratio of its cross-section (c) at the conductance limit to the area of incoming plum. However, at a given pumping capacity on the funnel, the pressure inside (P) is determined by the gas load that is proportional to c. Thus, the maximum feasible c depends on the highest usable P value. The performance and practicality of DTIMS also improves at higher pressure: in particular, the tube can be shortened without resolution loss. Again, the maximum pressure in DTIMS with front and/or back funnel interfaces is set by their limitations. The FAIMS resolving power also benefits from higher gas pressure (other factors being equal). Hence maximizing the operating pressure of ion funnels, ideally to 1 atm, is a key technological goal in the MS and IMS/MS field.
Physics of the ion focusing in Dehmelt potential requires a certain ratio of w to the ion-molecule collision frequency that depends on the ion species but is always proportional to pressure, hence w should be scaled with P. At a given gas temperature, effective focusing further requires a minimum potential depth that, by theory, scales as A2/w2. Therefore, raising the operating pressure also necessitates a proportional increase of A. An ion funnel is a capacitive bad and the power needed to drive it is proportional to electrical capacitance (c). Hence the realizable w and A values are limited by c, which thus should be minimized. First-generation funnels (with g=0.5 mm) developed in 1997-2002 had large capacitances that, with practical power supplies, limited w to ˜400 kHz and U to ˜40 V. These parameters allowed P up to ˜5 Torr depending on the species, which was close to the values in first stages of MS instruments with skimmer interfaces. Thus API/MS inlets were restricted to c˜0.3 mm2, resulting in large ion losses at the inlet faces and materially constraining the capabilities and utility of IMS/MS platforms. These devices still transmit ions an order of magnitude better than prior skimmer interfaces, and are now adopted in research and commercial MS systems as well as IMS/MS and FAIMS/MS platforms.
In 2nd-generation funnels developed since 2004, the capacitance was reduced 4-fold via a change of geometry and machining/assembly methods that minimized electrode surfaces and replaced the insulation between electrodes by air gaps with the lowest possible dielectric constant of 1. That has enabled a proportionally greater w˜2 MHz and U˜200 V, permitting similar increases of P to ˜30 Torr and c to ˜2 mm2 and higher, depending on the vacuum pumps and inlet capillary length. A single capillary with that large c would not desolvate ions completely and uniformly enough, but multiple (e.g., six) capillaries of regular diameter summing to c may be parallelized to reach high total flow while keeping the established desolvation regime. Large ion capture area and current capacity of such multicapillary inlets are of particular value with ESI emitter arrays. A higher pressure in the funnel similarly elevates that in the following MS chamber, increasing which by 5 times is generally untenable. Hence a high-pressure funnel was coupled to MS using an original (low-pressure) funnel. Such multicapillary inlet/tandem ion funnel interfaces (FIG. 2d) have improved the sensitivity of API/MS by ˜5 times compared to “standard” funnels, in proportion to the increase of P and gas intake via the inlet. However, losses are still large and further increase of the operating pressure and gas intake is desired. However, w and A could not be raised further within the existing paradigm of funnel assembly from individually machined macroscopic electrodes.
The field intensity in a gas is limited by the electrical breakdown threshold, which depends on the gas identity and pressure. While the rf voltages and thus A values in existing funnels can be raised using more powerful power supplies, a breakdown near the waveform peak would occur. Hence an approach to increase the funnel pressure by raising w and A must include the means to avoid breakdown.
An approach alternative to raising the funnel pressure is ESI in a sealed chamber at sub-ambient pressure. Such “SPIN” sources have been shown to work at a pressure as low as ˜30 Torr, allowing operation inside high-pressure funnels. While this virtually eliminates ion losses, the lower efficiency of ESI at 30 Torr offsets that, and the final ion yield is close to that using atmospheric-pressure ESI with multicapillary inlet/tandem ion funnel interface. Even if future ESI sources could hypothetically overcome that problem, the need for better ion focusing in IMS/MS and FAIMS/MS interlaces would remain and so would the need to increase the operating pressure of ion funnels, ideally to 1 atm.
The force of mutual Coulomb repulsion scales as the ion density squared and thus rapidly grows for stronger ion currents. The resulting space-charge expansions limit the resolving power of MS [in particular, orthogonal time-of-flight (o-ToF) MS] or IMS systems and their sensitivity, as ions exceeding the analyzer charge capacity are eliminated. Large ion flux gains provided by funnel interlaces known in the art already cause notable peak broadening in DTIMS, which would worsen as funnels at higher pressures deliver even greater ion currents. Hence reducing the space-charge effects is important for MS and IMS technology development and becomes increasingly topical as improvements of ion sources and front interfaces produce more intense ion beams.